1. Technical Field
The present disclosure relates to a switching power supply circuit.
2. Description of Related Art
Switching power supply circuits, exhibiting superior linear characteristics, consuming minimum power, converting electrical power efficiently, and performing steadily, are preferred for use in LCD TVs, displays, and other consumer devices.
FIG. 2 shows a commonly used switching power supply circuit. The switching power supply circuit 1 includes a first rectifying filtering circuit 10, a protection circuit 12, a transformer 13, a second rectifying filtering circuit 15, a pulse width modulation integrated circuit (PWM IC) 16, a rectifying diode 17, a transistor 18, and a resistor 19.
The PWM IC 16 includes a voltage input 161 receiving an operating voltage, and a pulse output 162 generating a pulse signal to a gate electrode of the transistor 18.
The first rectifying and filtering circuit 10 includes two inputs 101, 102 configured to receive an external alternating current (AC) voltage such as a 220V, a full-bridge rectifying circuit 103 configured to convert the 220V AC voltage to a first direct current (DC) voltage, a first filtering capacitor 104 configured to stabilize the first DC voltage, and a first output 105 configured to provide the first DC voltage to the transformer 13. Two inputs of the full-bridge rectifying circuit 103 serve as the two inputs 101, 102. A positive output of the full-bridge rectifying circuit 103 serves as the first output 105. A negative output of the full-bridge rectifying circuit 103 is grounded. The first filtering capacitor 104 is connected between the first output 105 and ground.
The transformer 13 includes a primary winding 131, a secondary winding 132, and an assistant winding 133. The primary winding 131 is electrically connected in parallel with the protection circuit 12. One terminal of the primary winding 131 is connected to the first output 105, and the other terminal of the primary winding 131 is connected to a drain electrode of the transistor 18. A source electrode of the transistor 18 is connected to ground via the resistor 19. A gate electrode of the transistor 18 is connected to the pulse output 162 of the PWM IC 16.
One terminal of the assistant winding 133 is connected to ground. The other terminal of the assistant winding 133 is connected to the voltage input 161 of the PWM IC 16 via the anode and the cathode of the rectifying diode 17 and a transistor (not labeled) in series.
The second rectifying and filtering circuit 15 includes a rectifying circuit 151, a second filtering capacitor 152, and a second output 153. One terminal of the secondary winding 132 is coupled to the second output 153 via the rectifying circuit 151. The other terminal of the secondary winding 132 is grounded. The second filtering capacitor 152 is connected between the second output 153 and ground.
The switching power supply circuit 1 operates as follows:
The external AC voltage is provided to the two inputs 101, 102 of the first rectifying and filtering circuit 10 and is converted to the first DC voltage by the first rectifying and filtering circuit 10. The first DC voltage is provided to the primary winding 131. The assistant winding 133 induces the primary winding 131, generates an operating voltage, and provides the operating voltage to the voltage input 161 of the PWM IC 16 via the rectifying diode 17. Thus the PWM IC 16 generates the pulse signal for switching the transistor 18 on or off. When the transistor 18 is switched on, a first current path is formed sequentially through the first output 105, the primary winding 131, the transistor 18, and the resistor 19. A first current is formed when the first DC voltage provided to the first output 105 is connected to ground via the first current path. The first current through the first current path increases linearly until electromagnetic induction generated in the primary winding 131 reaches a predetermined maximum threshold.
When the transistor 18 is switched off, energy stored in the primary winding 131 transfers to the secondary winding 132. Thus AC voltage across the secondary winding 132 is generated. The second rectifying and filtering circuit 15 converts the AC voltage to the second DC voltage, and provides the second DC voltage to a load circuit (not shown) via the second output 153.
Because the switching power supply circuit 1 includes numerous electric units, volume of the switching power supply circuit 1 is correspondingly large. Furthermore, cost of the PWM IC is high, increasing the expense of the switching power supply circuit 1.
It is thus desirable to provide a switching power supply circuit which can overcome the described limitations.